Modern LED (light emitting diode) display panels require higher grayscale to accomplish higher color depth and higher visual refresh rate to reduce flickering. For example, a 16-bit grayscale for a RGB LED pixel allows 16-bit levels (216=65536) for red, green, and blue LEDs, respectively. Such a RGB LED pixel is capable of displaying a total of 655363 colors. One of the methods commonly employed to adjust LED grayscale is Pulse Width Modulation (“PWM”). Simply put, PWM generates a series of voltage pulses to drive an LED. When the voltage of the pulse is higher than the forward voltage of the LED, the LED is turned on. Otherwise, the LED remains off. Accordingly, when the pulse amplitude exceeds a threshold, the pulse duration (i.e., pulse width) of the PWM signal decides the on-time and off-time of the LED. The percentage of on-time over the sum of on-time and off-time (i.e., a PWM cycle) is the duty cycle, which determines the brightness of the LED. Configurations and operations of an exemplary LED display system, which includes LED topology, circuitry, PWM engines, etc., are explained in detail in U.S. Pat. No. 8,963,811, issued Feb. 24, 2015, as well as in the co-pending U.S. patent application Ser. No. 15/901,712, filed Feb. 21, 2018.
Another parameter for an LED display is the grayscale value, which is the level of brightness of the LED display. In a 16-bit resolution LED display, the grayscale value ranges from 0 (complete darkness) to 65535 (maximum brightness), corresponding to duty cycles from 0% to 100%. When the grayscale value is low, the brightness level of an LED is low. Conversely, when the grayscale is high, the brightness level is also high. LED displays often experience performance issues at low grayscale values.
A further parameter for the LED display is its Grayscale Clock (“GCLK”) frequency, which is related to the maximum number of GCLK cycles (“GCLKs”) in a data frame and the refresh rate of the display. In addition, a frame rate is the number of times a video source feeds an entire frame of new data to a display in one second. The refresh rate of an LED display is the number of times per second the LED display draws the data. The refresh rate equals the frame rate multiplied by the number of segments.
One of the advantages of PWM is that power loss in the switching devices is low. When a switch is turned off, there is practically no current. When the switch is turned on, there is almost no voltage drop across the switch. As a result, power losses in both scenarios are close to zero. On the other hand, PWM is defined by the duty cycle, switching frequency, and properties of the load. When the switching frequency is sufficiently high, the pulse train can be smoothed and the average analog waveform can be recovered. However, when the switching frequency is low, the off-time of LED will be noticeable and appears as flickers to a viewer.
Scrambled PWM (“S-PWM”) modifies a conventional PWM and enables a higher visual refresh rate. To accomplish that, S-PWM scrambles the on-time in a PWM cycle into a number of shorter PWM pulses that sequentially drive each scan line. In other words, a total grayscale value is scrambled into a number of PWM pulses across a PWM cycle. In a conventional PWM scheme, there may be only one PWM pulse so that the LED is lit continuously for a period of time, leaving the LED unlit for the remainder of the time. In contrast, S-PWM allows the LED to emit light in consecutive short pulses in the PWM cycle so that the light pulses spread across the PWM cycle more evenly, avoiding or reducing flickers.
One PWM cycle has a number of GCLK cycles equaling 2 to the power of the number of control bits:Number_of_GCLKs=2NUMBER_OF_CONTROL_BITS.For example, a 16-bit grayscale has 65536 GCLKs. Note that the number of GCLKs in one PWM cycle equals its grayscale value at the maximum brightness, i.e., the maximum pulse width. In some S-PWM, the total number of GCLKs can be divided into MSB (most significant bits) and LSB (least significant bits) of grayscale cycles. Each PWM cycle is divided into a number of segments (or sub-PWM cycles) according to the following equation:Number_of_Segments=2NUMBER_OF_LSB.
For a video source of a 60 Hz frame rate and a PWM cycle length of 8000 GCLKs, one may divide the PWM cycle into 32 segments (LSB=5) so that each segment has a pulse duration of 250 GCLKs. A total of grayscale value of 1600 GCLKs therefore can be distributed into 32 segments at 50 GCLKs in each segment, potentially increasing the refresh rate up to 32 times. However, when the PWM pulse duration (i.e., pulse width) in the segment is shorter than the time it takes to raise the LED voltage above its forward voltage, the LED remains unlit. U.S. Pat. No. 9,390,647 provides a solution that extends the pulse duration by adding a fixed number of GCLKs to the pulse. However, such an S-PWM scheme results in large increments in the optical energy output at the low brightness level, as explained elsewhere in this disclosure. Other technical schemes may require a second power source to provide additional driving current to extend the pulse duration, adding complexity and costs to the electrical system for the LED display.
Accordingly, there is a need for new systems and methods that improves image quality of the LED display without the shortcomings of the existing technologies.